Current state-of-the-art engine controls rely almost exclusively on exhaust gas sensing to maintain the engine air-fuel ratio at a value that minimizes exhaust emissions. However, such sensors typically require heating for a significant period before the sensor is useful for control following a cold start. For this reason, engine fueling during engine starting and warm-up is generally performed based on an open-loop calibration. Until the engine has warmed up, a significant amount of the injected fuel puddles on the engine manifold walls instead of immediately vaporizing for ingestion in the cylinder. The puddled fuel evaporates over time, so that the fuel vapor actually ingested into the cylinder is generated in part from the injected fuel and in part from the puddled fuel. The rate at which the injected and puddled fuel quantities vaporize depends not only on temperature and pressure, but also on the fuel volatility, which may vary considerably from tank to tank. To complicate matters even further, any given fuel sample actually comprises hundreds of compounds of widely varying volatility. Under warmed-up conditions, it may be assumed that the puddled fuel (if any) comprises primarily low volatility compounds, the behavior of which may be reasonably accurately characterized. However, during cold-start and warm-up, the puddled fuel contains a wide variety of compounds, the behavior of which is difficult to accurately characterize. Thus, for a given amount of injected fuel, the quantity of fuel vapor actually delivered to the cylinder depends both on the fuel volatility and the evaporative characteristics of the fuel puddle.
The above-described variability forces design engineers to enrich the cold calibration--and generally to be less aggressive with spark retard used to assist catalyst heating--to insure that operating with low volatility fuel will not result in driveability problems. This enrichment to compensate for low volatility fuels causes the air/fuel mixture to be richer than optimum with high volatility fuel, resulting in higher engine-out hydrocarbon emissions than if the appropriate calibration for that fuel were used. Additionally, the less aggressive spark retard delays the onset of "light-off" of the exhaust catalyst. Thus, it is apparent that differences in fuel volatility adversely affect both emissions and driveability with conventional control strategies.
Accordingly, what is needed is a control method for accurately injecting fuel so that the actual air/fuel mixture in the engine cylinder more nearly corresponds to the desired air/fuel mixture, particularly during coldstart and warm-up conditions.